bepos -- determine the source position from a rough position list
bepos qpoe rough_fname bepos_tab bkden
BEPOS determines the maximum liklihood position for each peak identified by LPEAKS, using the original qpoe file. Signal-to-noise ratios in detect cells centered on the final positions are computed and positions with snr's greater than the user-defined threshold are identified as sources.
The algorithm uses the liklihood ratio parameter technique (cf. preprint by w. cash, and memos by s. murray, ssm-102-78 ans ssm-103-78). The method was used for the Einstein IPC and ROSAT HRI and given in detail in SAO Special Report 393 by Harnden, Fabricant, Harris, and Schwarz.
previous task to run: LPEAKS, BKDEN
- qpoe = "" prompt = Input qpoe file
Name of the input qpoe file. If ROSAT PSPC or Eintein IPC, PI bands must be respecified with the input qpoe filename. (ex. rp110590.qp[pi=52:201], if CELLMAP was run on the hard band)
ROSAT PSPC: soft band = [pi=11:41] hard band = [pi=52:201] broad band = [pi=11:235] Einstein IPC: soft band = [pi=2:4] hard band = [pi=5:10] broad band = [pi=2:10]
- rough_fname = "" prompt = input rough pos table
Required input filename. The file may be specified by supplying the rootname and a "_ruf.tab" will be appended. Each row in the table represents 1 peak position as determined by LPEAKS. BEPOS reads the following table columns of the input file:
x ------- x pixel position in physical coordinates y ------- y pixel position in physical coordinates cellx --- x detect cell size in arc-seconds celly --- y detect cell size in arc-seconds
- bepos_tab = "" prompt = output bepos table file
Required output filename. The file is written in table format. The file may be specified by supplying the rootname and a "_pos.tab" will be appended. Each row in the table represent 1 source. The column titles and definition appear in the table header and are as follows:
ra -------- right ascension in degrees dec ------- declination in degrees x --------- x pixel position in physical coordinates y --------- y pixel position in physical coordinates srccnts --- source strength (source counts only) in assumed point source. eq: alpha = integral of the PSF over the detect cell beta = integral of the PSF over the detect cell + background cell srcnts = ((16/9)cellcnts - framecnts)/ ((25/9)alpha - beta) bkcnts ---- background counts in the detect cell. eq: bkcnts = (alpha(cellcnts+framecnts) - Beta(cellcnts)) / ((25/9)Alpha - Beta) cellcnts -- counts at final position in a detect cell size box framecnts - bkdensity normalized to a detect cell size box pconf ----- position error in pixels at 90% confidence snr ------- signal-to-noise ratio C = cellcnts T = cellcnts + bkcnts SNR = ((25/9)*C - T ) / (sqrt(16/9**2-1)*C + T) cellx ----- x detect cell size in arc-seconds celly ----- y detect cell size in arc-seconds fsflag ---- false source probability flag: 1 means observed photon distribution is consistent with gaussian PRF for a point source 0 means observed photon distribution is inconsistent with gaussian PRF for a point source
- bkden = "" prompt = Intensity Levels
mean background density in counts/square arcminute. (computed by task BKDEN and written to this parameter as the default value by BKDEN.)
- (prf_sigma = 0.0) [real]
PRF Sigma of the field in arcseconds. When 0.0, the PRF sigma is computed by keying on Telescope, instrument (from input header) & bepos.energy in a PRF lookup table (bepos.prf_table) and using the coefficients that are returned to compute the PRF.
- (energy = 0.0) [real]
Energy (KeV), key look up in PRF coefficients table, coeffs are used to compute the Point Response Function Sigma.
- (prf_table = "xspatialdata$prfcoeffs.tab") [string]
PRF coefficients table. Entries for ROSAT PSPC/HRI & Einstein IPC/HRI. Lookup based on value of bepos.energy. Returns coefficients assiciated with a default lookup when energy = 0.0; returns coefficients closest to the energy when exact match is not found.
- (snr_thresh = 3.0) [real]
Minimum SNR threshold. Sources with computed SNR's below this threshold are rejected.
- (display = 2) [int]
Display info to screen. Integer of range 0-5. The higher the number more verbose the display.
- (clobber = no) [boolean]
Boolean flag specifying whether or not the output Table file can be overwritten, if it already exists.
- (max_iter = 50) [int]
Maximum iterations for convergence to best position.
- (max_conf_iterations = 20) [int]
Maximum iterations for confidence error calculations.
- (num_conf_lev = 2) [int]
Number of confidence levels that confidence errors are determined for below.
- (conf_= 2.3) [real]
The 68 percent confidence level for minimum c-statistic calculation.
- (conf_= 4.61) [real]
The 90 percent confidence level for minimum c-statistic calculation.
- (refine_limit = 45. ) [real]
Pixel limit between LPEAKS and BEPOS position. When the difference between the 2 positions is greater that this limit the source is thrown out.
- (optimum_radius= 1.8) [real]
Optimum radius sigma. Once the value of sigma has been determined, the maximum likelihood position is determined from events in a circular region of radius ROPT*sigma, where ROPT is this parameter (OPT_RADIUS_SIGMA). This region is calculated once for each input position to this task.
- (c_conv_epsilon = 1.0) [real]
Convergence minimum for c-statistic
- (s_conv_epsilon = 1.0) [real]
Convergence minimum for source
- (cenx_keywd = "CRPIX1" ) [string]
X center header keyword for off-axis angle calculation
- (ceny_keywd = "CRPIX2" ) [string]
Y center header keyword for off-axis angle calculation
de> bepos *** PI bands must be Respecified with input QPOE *** Input qpoe file: xdata$rh110267.qp Field background density in cts/arcmin**2 (0.001): 12.8558464 Input rough pos table: harlac24_ruf.tab Output bepos table file: . Input Qpoe: xdata$rh110267.qp bkden = 12.8558464 (cts/sq arc-min), 0.0008928 (cts/sq pixel) 2 Rough Position(s) Input src# ra dec physx physy cellcnts framecnts snr --------------------------------------------------------------------------- 1 22:08:38.4 45:31:24.2 2380.1 2829.1 95 67 5.3 2 22:08:41.4 45:44:33.4 2323.5 4407.7 1506 394 31.8 Writing 2 Sources to Output file: ./harlac24_pos.tab
Documentation on the detect algorithms and philosophy along with limitations in (help explain_detect ).
Documentation giving an overview of the detect package and run order of tasks. (help using_detect )